FIRST TRACES OF REPTILES. 53 



marine habits. And that the full importance of this pecu- 

 liarity of the early reptiles may be appreciated, the reader 

 must be made aware that modern reptiles have a ball-and- 

 socket form of the vertebrae that is, a convexity at the one 

 side fitting into the hollow of the adjacent bone ; but this 

 form only when they are mature animals, for in the embryotic 

 state of the crocodile and of the frog the form has been ascer- 

 tained to be biconcave, which gradually changes as the 

 animal approaches perfection. The teeth of the thecodonts 

 and palaeosaurs were fixed in distinct sockets, like those of 

 the modern crocodiles. In this respect, they were superior 

 to the modern varanians, the nearest living tribes, which have 

 the teeth imbedded in comparatively shallow cavities along 

 the bottom of a groove in the jaw. 



